(1) Field of the Invention
The present invention relates to a lighting method and apparatus for a high-pressure discharge lamp, and also to a high-pressure discharge lamp apparatus.
(2) Description of Related Art
In recent years, there have been active developments in projection display apparatuses such as liquid crystal projectors. A preferable light source for a projection display apparatus is a high-intensity light source that comes close to a point-source. For this reason, projection display apparatuses generally employ a high-pressure short-arc discharge lamp, such as a so-called high-pressure mercury lamp. A lighting circuit employed for operating such a high-pressure discharge lamp is, for example, an electronic ballast in full bridge configuration (supplying square wave voltage).
To meet the recent need for high-pressure discharge lamps that are comparable to a point-source, developments are underway for high-pressure discharge lamps having a shorter arc length. For example, in the case of a high-pressure mercury lamp with the nominal power of 150W, the electrode distance is shortened to 1.5 mm or less from the conventional electrode distance of 1.5-2.0 mm. Unfortunately, however, inventors of the present invention have noticed problems associated with such a lamp having the nominal power of 150W and the electrode distance of 1.5 mm or shorter. For example, in life tests conducted by the inventors of the present invention, some of such lamps exhibited abnormal blackening of the discharge tube within as short as 500 hours of aging.
An object of the present invention is to provide a lighting method for a high-pressure discharge lamp, a lighting circuit for a high-pressure discharge lamp, and a high-pressure discharge lamp apparatus all of which are free from such abnormal blackening of the discharge tube as described above.
The object stated above is achieved by a lighting method according to the present invention for a high-pressure discharge lamp. The high-pressure discharge lamp is composed of a discharge tube that is filled with mercury, a rare gas and a halogen material, and that has a pair of electrodes provided therein. The lighting method includes: a discharge shift detecting step of detecting that a discharge across the pair of electrodes has shifted to an arc discharge after a discharge starts and before the mercury fully evaporates; a voltage rise detecting step of detecting that a lamp voltage has risen to exceed a predetermined level after a shift to an arc discharge is detected and before the mercury fully evaporates; and a current reducing step of reducing a lamp current when the lamp voltage is detected in the voltage rise detecting step to exceed the predetermined level.
According to the lighting method of the present invention for a high-pressure discharge lamp, the lamp current is reduced when an abnormal discharge is detected. With this arrangement, the impact on the electrode roots is reduced, and thus there is an effect of suppressing blackening of the discharge tube that would otherwise caused by the abnormal discharge.